1. Field of the Invention
The invention concerns the preparation of X-ray pictures by means of photosensitive elements associated with a scintillator.
A scintillator is a substance with the property of being excited by X-rays and of emitting, in response to this excitation, a radiation with a wavelength in the visible or near visibile range. Photosensitive elements give an electrical signal when they receive this visible radiation, whereas they could not directly give an electrical signal in response to illumination by X-rays. Thus, an X-ray picture can be converted into a set of electrical signals representing this picture.
The invention shall be described only for this type of scintillator, which converts a range of very short wavelengths (X-rays) into a range of longer wavelengths (visible or near visible light), but it will be understood from the following description that the invention would be applicable to cases entailing the conversion of wavelengths which are different from those of the above-mentioned particular case (provided that there a scintillating substance available to perform the desired wavelength conversion).
2. Description of the Prior Art
The devices hitherto used for the electrical reproduction of X-ray pictures have either a pinpoint photosensitive detection element or a linear strip of photosensitive elements or, again, a rectangular matrix of elements. The element or elements are generally photodiodes or phototransisdors formed on an insulating substrate, and they are coated with a sheet of scintillating material which is either bonded to the upper surface of the photosensitive elements (on the X-radiation input side) or simply applied by pressure to this upper surface.
FIGS. 1a and 1b show two mutually orthogonal, lateral sectional views of a matrix of photosensitive elements associated in a standard way with a scintillating sheet.
Each photosensitive element consists, for example, of a PIN diode (a PN junction with a central, intrinsic part) and the matrix network has column conductors and row conductors having, at each row and column intersection, a PIN diode in series with a charge storage capacitor. The column conductors 12 consist of a metal deposited and photo-etched on an insulating substrate 10, preferably made of glass. The PIN diode is formed by deposition, on the column conductors, followed by etching, of three layers of amorphous silicon. These three layers are respectively doped as follows: P type (layer 14), intrinsic type (layer 16) and N type (layer 18). An insulating layer 20 is deposited on all the PIN diodes, and this layer forms the dielectric of the capacitor. A preferably transparent conductor is deposited on the insulating layer 20 and etched to form the row conductors 22 of the matrix. Finally, a scintillating sheet 24 is bonded or applied by pressure to the upper surface of the structure.
The radiation to be converted, namely, the X-radiation in the case of X-ray imaging, arrives by the top and strikes the upper surface 26 of the scintillator. If it were to arrive by the bottom, it would have to go through the glass substrate, the row conductors and column conductors and photosensitve diodes. The result thereof, chiefly because of the glass, would be losses which it is sought to avoid in order not to lose any part of the data, especially the low level data.
The thicker the scintillator, the more photons are emitted under X-radiation. Ultimately, for infinite thickness, all the X-radiation is absorbed and is converted into light photons. But conversely, if the scintillator is too thick, the photons emitted from the side opposite the photosensitive detector will be absorbed in their path through the scintillator (which is not completely transparent) and would not reach the photosensitive detector. These photons will be unprofitably lost. A compromise thickness should generally be found. This thickness is in the range of about 200 micrometers.
Besides, for scintillators made in the form of compacted powder sheets, the definition is all the smaller as the sheet is thicker. For, the photons are emitted in all directions and, in fact, the photons emitted at a distance d from the photosensitive element produce a light spot with a density which is distributed according to a Gaussian relationship, the approximate useful diameter of this spot being substantially equal to the distance d. Here again, an excessive thickness has to be avoided and a compromise has to be found for the pair of parameters comprising definition and sensitivity.
The invention results from the observation that, in the structure of FIGS. 1a and 1b, that side of the scintillator which is first struck by the X-rays is the side 26 and not the side in contact with the photosensitive diodes. The result of this is that the greatest number of light photons is emitted on the side which is struck first.
Now, it is precisely those photons emitted on the further side of the surface of the photosensitive detector that tend to limit both sensitivity (because these photons do not all succeed in crossing the sheet) and definiion (because they are emitted far from the photosensitive surface).
An object of the invention is to improve both the definition and the sensitivity of the device, and to do so without it being necessary to use scintillators with very particular crystalline structures, deposited in the form of vertical needles (caesium iodide scintillators) providing high definition.
Another object of the invention is to facilitate the manufacture of the device in preventing accidents due to the brittlenes of the scintillating sheet which is very thin (about 200 microns).